1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure sensing apparatuses or sensors. More particularly, the invention relates to a direct-type tire inflation pressure sensing apparatus in which a transmitter is supplied with electrical power by a transponder method.
2. Description of the Related Art
Conventional direct-type tire inflation pressure sensing apparatuses generally include a transmitter and a receiver. (For example, Japanese Patent No. 3212311, an English equivalent of which is U.S. Pat. No. 5,602,524, discloses such an apparatus.)
The transmitter is directly installed to a wheel of a vehicle and includes a pressure sensor working to sense an inflation pressure of a tire mounted on the wheel. The transmitter is configured to send out a pressure signal representative of the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes an antenna. The receiver is configured to receive the pressure signal from the transmitter through the antenna and determine the inflation pressure of the tire based on the received pressure signal.
In the above arrangement, the transmitter is generally supplied with electrical power by a battery provided on the wheel of the vehicle. On the other hand, investigations have recently been made to supply electrical power to the transmitter by a transponder method.
According to the transponder method, the receiver works to send out through the antenna thereof a radio wave for electrical power charge toward the transmitter, thereby charging a charging unit of the transmitter with electrical power. When the charging unit of the transmitter is completely charged with electrical power, the transmitter is then activated by the charged electrical power and works to send out the pressure signal toward the receiver.
Such a transponder method is generally used in the field of wireless ID cards. The application of the method to direct-type tire inflation pressure sensing apparatuses provides an advantage of supplying electrical power to the transmitter without any battery provided on the wheel.
In a direct type tire inflation pressure sensing apparatus based on the transponder method, the receiver generally has high sensitivity, and thus can accurately recognize a signal received thereby even when the signal has low level. Accordingly, the receiver can receive and accurately recognize the pressure signal sent out from the transmitter regardless of the angular potion of the transmitter that rotates together with the wheel.
On the other hand, the radio wave for electrical power charge received by the transmitter is required to have a high level so as to be able to charge the charging unit of the transmitter with electrical power.
However, when the transmitter rotates together with the wheel, it is impossible for the transmitter to receive the radio wave for electrical power charge with high level at any angular position thereof.
Especially, when the maximum level of the radio wave sent out from the receiver is restricted under regulations, a possible range of angular position of the transmitter for charge, within which the level of the radio wave for electrical power charge received by the transmitter is above a necessary level for charging the charging unit with electrical power, is very small when the transmitter rotates together with the wheel.
FIGS. 6A–6B illustrates the position of the transmitter relative to the antenna of the receiver. In the figures, the antenna J1 of the receiver is installed to a fender J2 of the vehicle body such that a vertical centerline C—C of the wheel J3 passes through the antenna J1.
In this case, according to the results of an experimental investigation, the possible range of angular position of the transmitter for charge (to be referred to as possible range for charge hereinafter) may be defined as the angular range A-O-B of about 90°, which spreads out from the shaft of the wheel J3 and is symmetrical with respect to the vertical centerline C—C.
In other words, when the transmitter J4 is within the above angular range A-O-B, the transmitter J4 receives the radio wave for electrical power charge from the antenna J1 with high level, so that the charging unit of the transmitter J4 can be charged with electrical power.
Moreover, when the radio wave for electrical power charge sent out from the receiver and the pressure signal sent out from the transmitter J4 have frequencies within the same frequency band, it is very difficult for the receiver to send out the radio wave while receiving the pressure signal and demodulating the received pressure signal. In other words, even if the receiver can receive the pressure signal from the transmitter J4 while sending out the radio wave, the level of the pressure signal received by the receiver is so low that the receiver cannot demodulate the received pressure signal.
Thus, it is necessary for the transmitter J4 to be triggered to send out the pressure signal when the receiver stops sending out the radio wave for electrical power charge, so that transmissions of the radio wave for electrical power charge and the pressure signal can be prevented from being made simultaneously.
However, it is very difficult for the receiver to determine whether the transmitter J4 rotates in the possible range for charge. Also, it is very difficult for the transmitter J4 to determine when the receiver stops sending out the radio wave for electrical power charge.
Accordingly, reliable transmissions of the radio wave for electrical power charge and the pressure signal between the receiver and the transmitter J4 are very difficult to be realized.
Further, when the vehicle is running, the position of the antenna J1 of the receiver relative to the shaft of the wheel J3 varies depending on the road condition and vibrations. Furthermore, different antennas J1 may have different performance characteristics depending on the manufacturing conditions.
Accordingly, it is very difficult to preset a necessary charging time for completely charging the charging unit of the transmitter with electrical power.